The present invention relates to a method of driving a stepping motor and more particularly, to a technique of restraining vibration of the stepping motor during drive thereof.
One of conventional methods of driving a stepping motor is disclosed, for example, in JP-A 4-58790. In this reference, upon switching of a waveform of a drive signal for driving stepwise a stepping motor, the on-duty ratio of the drive signal is gradually changed by pulse width modulation (refer hereafter to as PWM) control so as to retrain a settling due to sudden step rotation. By this, the stepping motor has reduced vibration noise during step drive.
However, the above conventional method of driving a stepping motor produces the following inconveniences:
PWM control as adopted in the conventional method is designed to gently carry out a rise of drive current output to the stepping motor by restricting a time duration of supply of the drive signal, so that an average of drive current is lowered at each drive step, resulting in lowered torque of the stepping motor during PWM control. If source voltage of a stepping motor drive circuit is decreased during PWM control, drive torque is lowered further, resulting in a possible erroneous operation of the stepping motor due to the relationship between load and drive torque. By way of example, when the stepping motor serves as an actuator for driving damping-force-characteristic varying means of a variable-damping-force-type shock absorber for a motor vehicle, the damping-force-characteristic varying means undergo a load based on a fluid force. Thus, if source voltage of the stepping motor drive circuit is lowered during PWM control due to a great variation in source voltage according to a service of the other electric devices in the motor vehicle, etc., and/or the stepping motor undergoes a great load suddenly, the stepping motor is out of good operation, resulting in erroneous control. It is noted that a variation in source voltage can be corrected by providing a constant current circuit based on feedback control of current, which produces, however, another inconvenience of a cost increase due to the complicated circuit structure and increased number of parts.
Moreover, according to the conventional method, the stepping motor is driven smoothly only in an initial portion of each drive step, but driven stepwise in accordance with the drive signal in the same way as in the prior art, failing to fully eliminate vibration of the stepping motor during step drive thereof.
When driving the stepping motor for driving damping-force-characteristic varying means of a variable-damping-force-type shock absorber, a waveform of damping force has disarrayed portions due to intermittent switching of a damping-force characteristic, which is therefore increased with an enlargement of the width of each drive step of the stepping motor 3.
It is, therefore, an object of the present invention to provide a method of driving a stepping motor which enables a prevention of an erroneous operation of the stepping motor and a restraint of occurrence of vibration of the stepping motor during step drive thereof.